1. Field of the Invention
The present invention relates to a method and apparatus for monitoring the operation of a reciprocating liquid injection system as for the injection molding of molten metal, such as molten aluminum, and has for its objective the provision of a method and apparatus wherein an undesirable operating condition can be promptly detected.
2. Description of the Prior Art
It is known to produce various articles by die casting and like methods by injecting a molding liquid, i.e. molten metal such as aluminum, into molds with an injection system. According to the injection die casting method, a molding liquid, e.g. a molten metal, is delivered within an injection sleeve communicating with an inlet of the mold and is injected into a mold cavity by a plunger which is reciprocated by a hydraulic operating cylinder. This method is widely employed for the mass production of machine parts and other articles of daily use because it is possible with this method to cast continuously and automatically shaped articles of high dimensional accuracy. However, during the repeated operation of this kind of injection mechanism, molten metal can adhere to the interior of the sleeve or the plunger head or tip slidably reciprocating therein, resulting in a reduction in the working clearance between the sleeve and head, which causes a phenomenon known as "dragging" to occur between the sleeve and plunger head. Continued operation in the system while in this state can lead to serious problems, for example, the sliding surfaces of the sleeve and head can be injured, and as the dragging phenomenon tends to grow worse gradually, the quality of the molded products suffers and the molding mechanism can be damaged intensively. Therefore, in order for the injection molding to be carried out smoothly and properly, it is important that when such an operating failure occurs, this phenomenon be detected reliably at an early stage of its existence so that appropriate action can be taken to eliminate its cause. However, since this phenomenon takes place inside the injection sleeve while the casting operation is performed automatically by the hydraulic drive cylinder, it is difficult to precisely detect its occurrence externally and, consequently an acceptable solution to this problem has not yet been devised.